The formation, consequences, and mechanism of repair of DNA-protein cross-links are being studied in mouse cells and human fibroblasts, including cells from xeroderma pigmentosum (XP) patients and normal individuals. The mechanism of repair of cross-links induced with 20 MuM trans-platinum(II) diamminedichloride is currently emphasized. It is known that DNA-protein cross-links are induced by numerous agents, including x-rays, ultraviolet light, visible fluorescent light, and a wide variety of chemical carcinogens such as benzopyrene, methylmethane sulfonate, and AAF. These lesions induced by trans-platinum have been reported to result in transformation of 3T3 and 10T-1/2 mouse cells, and they are repaired by the nucleotide excision pathway. However, because L1210 mouse cells are relatively deficient in nucleotide excision repair compared to normal human fibroblasts, but are at least as competent at repair of these lesions, the mechanism of repair was investigated further. The approach used was to first compare the DNA-protein cross-link repair rates of two human cell lines deficient in nucleotide excision repair, one slowly proliferating (XP20S, group A) and the other rapidly proliferating (SV40-tranformed XP20S); then, the effect of the metabolic inhibitors, cycloheximide and aphidicolin, on the repair of DNA-protein cross-links was examined. Finally, the rates of DNA repair and DNA replication during logarithmic growth and their relation to the cell cycle were studied. From the results of these experiments, we conclude the following: (1) A pathway in addition to the nucleotide excision mechanism can repair DNA-protein cross-links induced by transplatinum. (2) Cell cycling is necessary for activation of this pathway. (3) DNA synthesis per se is not essential for this repair. (4) The pathway seems to be activated in late G1 or early S phase of the cell cycle.